Image processing apparatus generally have an internal memory for storing image data from a hard disc or like external memory or an image reader and are adapted to process the image data stored in the internal memory in various modes and deliver the processed data to a display or printer.
For enlargement, the stored image is read out from the internal memory, enlarged and stored again in the internal memory at a specified address. When the read-out image data is temporarily stored in a buffer memory, the image data can be enlarged when the data is stored in the buffer memory or when it is returned from the buffer memory to the internal memory.
The buffer memory to be used has a capacity to store a line or several lines of data. Although it is possible to use a buffer memory having a capacity to store an amount of data to be displayed which can include an entire screen of the display at a time, an increased cost will then result.
When a buffer memory with a capacity of one line is used, image data is read out from the internal memory simultaneously with rewriting of the enlarged data. It is therefore likely that the original stored image will be broken owing to the relation between the location of the original image in the internal memory and the location where the enlarged image is written.
This will be described with reference to FIG. 1, which shows the storage area of the internal memory two-dimensionally in a state in which the stored image is to shown on a display. It is now assumed that the original image data is stored in region A and is to be stored in region B as enlarged twofold. To enlarge the image, the data is read out first from the uppermost line A.sub.1 of region A, then enlarged and stored first in the uppermost line B.sub.1 of region B. The data in lines A.sub.1 to A.sub.i-1 of region A can be properly restored in lines B.sub.1 to B.sub.i-1 of region B. At this time, lines A.sub.i-2 and A.sub.i-1 coincide with line B.sub.i-1 in the internal memory. When line A.sub.i is subsequently enlarged and written in line B.sub.i, line A.sub.i+1 from which the original data is to be read out is also replaced by the enlarged data. Consequently, the enlarged image portion in lines A.sub.i+1 et seq. differs from the original image portion.
The process of enlargement will be described next. Enlargement is accomplished by an electrical process in which the ratio of the speed at which image data is written in the buffer memory to the speed at which the image data is read from the buffer memory is varied. Suppose the writing speed is 256 clock pulses (M) per unit time, and the reading speed is N clock pulses (64.ltoreq.N.ltoreq.256). The magnification of M/N=1X to 4X is then variable in 193 steps. In this case, however, the magnification is dependent on the value N, which is entered by the user, so that it is difficult for the user to recognize the actual magnification. Further the magnification obtained varies stepwise with the value N, and an intermediate magnification value is not available. For example, when a magnification of 1.700X is required, an N value of 150 or 151 gives a magnification approximate to this value, i.e., 1.707X or 1.695X.